--- /dev/null
+# selector-type-no-unknown
+
+Disallow unknown type selectors.
+
+```css
+ unknown {}
+/** ↑
+ * This type selector */
+```
+
+This rule considers tags defined in the HTML, SVG, and MathML specifications to be known.
+
+## Options
+
+### `true`
+
+The following patterns are considered violations:
+
+```css
+unknown {}
+```
+
+```css
+tag {}
+```
+
+The following patterns are *not* considered violations:
+
+```css
+input {}
+```
+
+```css
+ul li {}
+```
+
+```css
+li > a {}
+```
+
+## Optional secondary options
+
+### `ignore: ["custom-elements", "default-namespace"]`
+
+#### `"custom-elements"`
+
+Allow custom elements.
+
+The following patterns are considered violations:
+
+```css
+unknown {}
+```
+
+```css
+x-Foo {}
+```
+
+The following patterns are *not* considered violations:
+
+```css
+x-foo {}
+```
+
+#### `"default-namespace"`
+
+Allow unknown type selectors if they belong to the default namespace.
+
+The following patterns are considered violations:
+
+```css
+namespace|unknown {}
+```
+
+The following patterns are *not* considered violations:
+
+```css
+unknown {}
+```
+
+### `ignoreNamespaces: ["/regex/", "string"]`
+
+Given:
+
+```js
+["/^my-/", "custom-namespace"]
+```
+
+The following patterns are *not* considered violations:
+
+```css
+custom-namespace|unknown {}
+```
+
+```css
+my-namespace|unknown {}
+```
+
+```css
+my-other-namespace|unknown {}
+```
+
+### `ignoreTypes: ["/regex/", "string"]`
+
+Given:
+
+```js
+["/^my-/", "custom-type"]
+```
+
+The following patterns are *not* considered violations:
+
+```css
+custom-type {}
+```
+
+```css
+my-type {}
+```
+
+```css
+my-other-type {}
+```